1. Field of the Invention
The present invention relates to a magnetic memory device and, more particularly, to a Bloch line memory device which employs as an information carrier a vertical Bloch line that is a microstructure of domain wall present in a magnetic wall which surrounds a stripe magnetic domain in a thin ferromagnetic film (thin magnetic garnet film).
2. Description of the Related Art
Like magnetic bubble memory devices, Bloch line memory devices use a magnetic garnet film as a memory medium film. However, these two types of memory device greatly differ from each other in terms of the information storing method. More specifically, in the conventional magnetic bubble memory devices, the presence and absence of a magnetic domain are arranged to correspond to information "1" and "0", respectively, whereas, in the Bloch line memory devices, the presence and absence of a vertical Bloch line pair in the magnetic wall surrounding a stripe magnetic domain formed by stretching a bubble domain are arranged to correspond to information "1" and "0", respectively. FIG. 1 shows the arrangement of a Bloch line memory device according to a prior art. In FIG. 1, the upward arrows 1a and 1b within a strip magnetic domain 1 denote the direction of magnetization, while the arrow 101 on the center line of a magnetic wall 2 denotes the direction of magnetization of the center line, and the arrow 102 which is perpendicular to the center line of the magnetic wall 2 denotes the direction of magnetization of the center of each vertical Bloch line 3 (hereinafter referred to simply as "Bloch line"). A portion 4 where a pair of Bloch lines 3 are present corresponds to the information "1", whereas, a portion 6 where no Bloch line 3 is present corresponds to the information "0".
The Bloch line that is employed as an information carrier is a microstructure of domain wall which is present in the magnetic wall 2 surrounding the stripe magnetic domain 1. The Bloch line 3 is stably present in the magnetic wall 2 and freely movable therein. Accordingly, if a multiplicity of stripe magnetic domains 1 are disposed in parallel at predetermined positions and Bloch lines 3 are allowed to be present within the magnetic walls 2, the Bloch lines 3 show behavior such as that of bubble domains moving through minor loops of a magnetic bubble memory device. Therefore, a Bloch line memory device can be used to arrange a memory device in the form of a shift register in the same way as in the case of a magnetic bubble memory device.
The presence of Bloch line has been known for a long time and it has been experimentally and analytically proved that the presence of Bloch lines decreases the speed of movement of magnetic domains. For this reason, in magnetic bubble memory devices in which magnetic domains must be moved, a bubble domain which includes a Bloch line is called a hard bubble and measures have been taken to prevent generation of Bloch lines. In contrast, in Bloch line memory devices, the presence of Bloch lines is positively utilized.
The physical size of a Bloch line is about 1/10 of the width of a stripe magnetic domain in which it is present, and a multiplicity of Bloch lines are allowed to be present within a single stripe magnetic domain. For example, in the case of a magnetic garnet film with a stripe magnetic domain width of 1 .mu.m which is presently developed for magnetic bubble memory devices, about 5.times.10.sup.8 Bloch lines are allowed to be present per cm.sup.2. Accordingly, if two Bloch lines are paired to form an information carrier, it is possible to produce a memory device of 256 Mbit/cm.sup.2 in capacity.
There is the following reason for Bloch lines to enable realization of a large memory capacity in addition to the advantage that the size thereof is very small. More specifically, in the magnetic bubble memory, the in-plane field is rotated to propagate information carriers, whereas, in the Bloch line memory, a perpendicular field is employed to propagate information. Accordingly, the propagation track pattern is planar and simple, which facilitates achievement of a high density of the device.
As has been described above, Bloch lines are freely movable around a stripe magnetic domain and capable of storing information. However, it is necessary in order to form a memory device to realize writing and reading of information.
As to a method of writing information, it is generally well known practice to supply a current to conductors disposed at an end of a stripe magnetic domain so as to apply a local field to the end portion of the stripe magnetic domain to thereby invert 180.degree. the direction of magnetization. In other words, it may be considered that the state of magnetization denoted by "0" in FIG. 1 is inverted to the state of the region "1". At this time, magnetization continuously changes at the boundary between an inverted region and a non-inverted region and therefore a state wherein the direction of magnetization is at 90.degree. with respect to the magnetic wall is produced. This is a Bloch line. It should be noted that since a pair of Bloch lines are infallibly produced, information "1" and information "0" are arranged to correspond to the presence and absence of a pair of Bloch lines.
Reading of information is carried out after conversion of the presence or absence of a vertical Bloch line pair into the presence or absence of a bubble domain. The conversion from Bloch lines into bubble domains is effected using the method disclosed by Konishi in IEEE Trans, MAG19, No. 5 (1983), pp. 1838-1843. This method will be explained with reference to FIGS. 2A to 2D. When a Bloch line 3 is present in a magnetic wall 2 surrounding a stripe magnetic domain 1, the direction of magnetization within the magnetic wall 2 is inverted at the Bloch line 3, that is, the direction of magnetization at one side of the Block line 3 is reverse to that at the other side of the Bloch line 3. Such a change in the structure of domain wall leads to a difference in terms of easiness with which the end portion of the magnetic domain is chopped between the case where one Bloch line 3 has moved to the end portion of the stripe magnetic domain 1 as shown in FIG. 2A and the case where no Bloch line is present at the end portion of the stripe magnetic domain 1 as shown in FIG. 2C. More specifically, only when one Bloch line is present at the end portion of the stripe magnetic domain 1 as shown in FIG. 2A, a bubble domain 8 can be chopped off from the end portion of the stripe magnetic domain 1 as shown in FIG. 2B by supplying a predetermined current Ic to parallel conductors (chopping conductors) 7 which are provided over the end portion of the stripe magnetic domain 1. When no Bloch line 3 is present at the end portion of the stripe magnetic domain 1 as shown in FIG. 2C, the bubble domain cannot be chopped off even if the current Ic is supplied to the chopping conductors 7. If the bubble domain 8 chopped off as shown in FIG. 2B is propagated and converted into an electric signal by a method similar to that used for the major line of a bubble memory device, information corresponding to the presence of a Bloch line can be read out.
Sections that realize the respective functions of writing, storing and reading information as described above are formed on the same one device to realize a Bloch line memory device.
It should be noted that the above-described writing and reading operations are described in U.S. Pat. No. 4,583,200.
In the above-described prior art, chopping of a new magnetic domain is effected by supplying two parallel conductors with current pulses reverse to each other. However, a new bubble magnetic domain 8 is chopped off only when a Bloch line 3 is present at a longitudinal end portion of the stripe magnetic domain (memory section) which intersects the parallel conductors 7 at substantially right angles. The presence and absence of the bubble domain 8 correspond to the presence and absence of a Bloch line which are, in turn, arranged to correspond to information "1" and "0", respectively. The presence of a bubble domain 8 is detected by a detecting method employed in the existing magnetic bubble memory devices. In this way, information carried by a Bloch line is read out.
However, the above-described prior art mentions nothing about the waveform of current pulses supplied to the parallel conductors to chop off a magnetic domain. Further, the experiment conducted by the present inventors has revealed that a mal-operation is likely to occur when a magnetic domain is chopped off simply by supplying rectangular pulses to the parallel conductors 7 (hereinafter referred to as "chopping conductors") and the operating range thereof is narrow. The contents of the experiment will be explained hereinunder with reference to the drawings.
FIG. 3 is a chart showing the waveform of a current pulse i'.sub.R supplied to chop off the stripe magnetic domain 1. The pulse rise time is 3 nsec, while the pulse width is 40 nsec, and the pulse fall time is 3 nsec. As has been described with reference to FIG. 2, the easiness with which the end portion of the stripe magnetic domain 1 is chopped off in the case where a Bloch line 3 is present at the end portion of the stripe magnetic domain 1 (FIG. 2A) differs from that in the case where no Bloch line 3 is present (FIG. 2C). More specifically, when a Bloch line 3 is present at the end portion of the stripe magnetic domain 1 (FIG. 2A), the directions of magnetization 5 of the opposing magnetic walls 2 are the same as each other. On the other hand, when no Bloch line 3 is present (FIG. 2C), the directions of magnetization 5 of the opposing magnetic walls 2 are opposite to each other. For this reason, when the opposing magnetic walls 2 come close to each other in the process of chopping off a magnetic domain, there is a difference in the exchange interaction taking place between the magnetizations of the magnetic walls. Therefore, a magnetic domain can be chopped off only when a Bloch line 3 is present (i.e., the directions of magnetizations of the opposing magnetic walls 2 are the same). By utilizing this nature, it is possible to convert the presence and absence of a Bloch line into the presence and absence of a bubble magnetic domain (i.e., a magnetic domain chopped off from the stripe magnetic domain). However, an experiment conducted by the present inventors has revealed that there is a case where the phenomenon which is opposite to the above occurs. This will next be explained with reference to FIG. 4.
Like FIG. 2, FIG. 4 shows a case where a Bloch line 3 is present at the end portion of a stripe magnetic domain 1 [FIGS. 4(a), 4(b) and 4(c)] and a case where no Bloch line is present [FIGS. 4(d), 4(e) and 4(f)]. The present inventors have found a possibility of a mal-operation that the stripe magnetic domain 1 cannot be chopped off when a Bloch line 3 is present but it is chopped off when no Bloch line 3 is present instead. This mal-operation is caused due to the fact that, when the current pulse i'.sub.R shown in FIG. 3 is applied (t=t.sub.2), new Bloch lines 3-a and 3-b are generated as shown in FIG. 4(b), thus causing the direction of magnetization 5 of the magnetic wall 2 to be inverted. If the direction of magnetization 5 is inverted, the degree of easiness with which the stripe magnetic domain is chopped off and which is determined according to whether a Bloch line 3 is present or absent is inverted, as will be clear from FIGS. 5(b) and 5(e). For this reason, the above-described mal-operation is caused. This is because the shrinkage of the stripe magnetic domain 1 and the chopping operation take place for a short period of time (substantially simultaneously) since the rise of the current pulse i'.sub.R is steep.
To prevent the mal-operation, it suffices to cause the stripe magnetic domain to shrink under conditions where the Bloch lines 3-a and 3-b are not generated. However, it is necessary to set the current value at a relatively high level in order to chop off the magnetic domain by means of a current pulse in the prior art, and Bloch lines are generally generated at a current value approximately equal to the value of the current pulse for chopping. Accordingly, there is a considerably high probability that a mal-operation will take place at the time of chopping off a magnetic domain.